The invention is directed to a regeneration agent for salt baths for the carburization of iron and steel parts, the regeneration agent being in the form of polymeric organic compounds and a process for their production.
Salt baths for the carburization of iron and steel parts in the hardening art generally consist of a mixture of alkali cyanide (e.g., sodium cyanide or potassium cyanide) as the active carburization material, barium chloride as the carrier melt, and alkali carbonate (e.g., sodium carbonate and potassium carbonate). They are operated at a temperature of 800.degree. to 950.degree. C. At this temperature, carbon preferentially diffuses into the surface of the workpieces hung for about 1 to 5 hours in the melt. By subsequent quenching of the carburized workpieces, there is produced a high edge or boarder hardness and high resistance to wear. However, during the operation of the salt bath, there occurs a gradual oxidation of the cyanide by the oxygen of the air to carbonate, which is inactive for the carburization process.
Because the baths become inactive through this, until now the original bath composition must again be produced from time to time by addition of cyanide or cyanide containing salt mixtures. For each regeneration, a part of the salt must be carried out of the bath and discarded as highly toxic salt. This method of operation, however, has the disadvantage that highly toxic waste salt must be discarded and the toxic cyanide must be stored as regeneration agent. Therefore, it has already been proposed (German Pat. No. 2310815 and related Beyer U.S. Pat. No. 4,019,928) to use for the regeneration of salt baths for the nitriding of workpieces polymeric triazine compounds, polymeric hydrocyanic acid, and polymeric carboxylic acid amides (German Pat. No. 2409285). These additives have the advantage that they are nontoxic. In principle, they are also usable for carburization. With addition of these compounds, the carbonate formed in the salt bath crucible through the oxidation and unsuitable for the carburization itself is changed back into carburization active cyanide. In this manner of operation, therefore, there is eliminated the draining of waste salts and the storage of cyanide.
However, the above-mentioned regeneration agent in practice is employed preponderantly only for the regeneration of nitriding salt baths at temperatures of around 580.degree. C. In the use of this material for the regeneration of carburizing baths at 800.degree. to 950.degree. C., there occur a series of disadvantages, which until now have prevented their employment in that art.
Thus, in using melon or polymers derived from urea, there are produced only small amounts of the carburizing active cyanide, and instead large amounts of cyanate which only partially and slowly decomposes to cyanide and is undesired in carburizing salt baths because it negatively influence the carburization action and leads to edge oxidation on the steel. Besides through the decomposition of the cyanate at the high temperatures of the carburizing baths, there occurs a strong foaming of the salt bath which can lead to overflowing of the melt. Furthermore, the reaction of this regeneration agent with the melt proceeds extremely vigorously.
In using the known polymeric hydrocyanic acid (azulminic acid), indeed the above difficulties occur only to a limited extent. However, instead there arise other problems. Thus, azulminic acid in the reaction with carbonate forms in addition to cyanide considerable amounts of carbon through which there is built up a dense bath covering which increases the difficulty of the regeneration. The yield of cyanide is, therefore, unsatisfactory. Besides, the production of the polymeric hydrocyanic acid requires extensive safety procedures and great apparatus expense since it must start from the highly toxic hydrogen cyanide.
Therefore, it was the problem of the present invention to find regeneration agents in the form of polymeric organic compounds for salt baths for carburizing iron and steel parts, which agents effect a practically complete conversion of carbonate into cyanide, cause no excess foaming of the salt bath, form no carbon residues, and are safe to produce.